Interventional medical systems, devices, and components thereof

ABSTRACT

A fixation member component, for example, employed by a relatively compact implantable medical device, includes a plurality of fingers; each finger includes a first segment extending from a fixed end of the corresponding finger, and a second segment extending from the corresponding first segment to a free end of the corresponding finger. Each first segment is elastically deformable from a relaxed to an extended condition, and from the relaxed to a compressed condition, and includes a peripheral portion and a central cut-out portion, framed by the peripheral portion. In the compressed condition, a free tip of the cut-out portion of some or all of the fingers may lodge against opposing tissue surfaces, via a spring force of the compressed fingers. Each second segment and cut-out portion is preferably configured to prevent penetration thereof within tissue at the implant site.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to the U.S. Provisional PatentApplication Ser. No. 62/041,940, which was filed on Aug. 26, 2014 and ishereby incorporated by reference in its entirety. The presentapplication is related to the co-pending and commonly assigned U.S.patent application Ser. No. 14/518,261, which is filed concurrentlyherewith and entitled INTERVENTIONAL MEDICAL SYSTEMS, DEVICES, ANDMETHODS OF USE, and which is incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure pertains to interventional medical systems, andmore particularly to relatively compact implantable medical devicesthereof and associated components.

BACKGROUND

The traditional implantable cardiac pacemaker includes a pulse generatordevice to which one or more flexible elongate lead wires are coupled.The device is typically implanted in a subcutaneous pocket, remote fromthe heart, and each of the one or more lead wires extends therefrom to acorresponding electrode, coupled thereto and positioned at a pacingsite, either endocardial or epicardial. Mechanical complications and/orMRI compatibility issues, which are sometimes associated with elongatelead wires and well known to those skilled in the art, have motivatedthe development of implantable cardiac pacing devices that are whollycontained within a relatively compact package, the entirety of which isconfigured for implant in close proximity to the pacing site. FIG. 1 isa schematic diagram that shows potential cardiac implant sites for sucha device, for example, within an appendage 102 of a right atrium RA,within a coronary vein CV (via a coronary sinus ostium CSOS), or inproximity to an apex 103 of a right ventricle RV. An implantingphysician may employ a standard guiding catheter (not shown) to delivera relatively compact medical device to any one of the three exemplarysites, for example, according to methods known in the art ofinterventional cardiology, by maneuvering the catheter, with the deviceloaded therein, up through the inferior vena cava IVC and into the rightatrium RA. However, a co-pending and commonly assigned U.S. PatentApplication having the Ser. No. 14/039,937 discloses a moresophisticated delivery tool, which the operator may employ, in lieu ofthe standard guiding catheter, to deliver and to fix the device at thedesired implant site.

SUMMARY

A relatively compact implantable medical device, according toembodiments disclosed herein, includes a fixation member formed by aplurality of fingers mounted around a perimeter of a distal end of ahousing of the device, wherein each finger includes a first segmentextending from a fixed end of the corresponding finger, and a secondsegment extending from the corresponding first segment to a free end ofthe corresponding finger. The first segment of each finger iselastically deformable from a relaxed condition to an extendedcondition, and from the relaxed condition to a compressed condition, andeach first segment includes a peripheral portion and a central cut-outportion, which is framed by the peripheral portion so that an insideedge of the peripheral portion is spaced apart from an outside edge ofthe central cut-out portion. The central cut-out portion of the firstsegment of each finger extends from a fixed end thereof to a free tipthereof, wherein the fixed end is integral with the peripheral portionand located in proximity to the fixed end of the corresponding finger.The second segment of each finger extends in a proximal direction andoutward from the device housing, when the corresponding first segment isin the relaxed condition, and extends distally from the distal end ofthe device housing, when the corresponding proximal end is in theextended condition. According to some embodiments, the second segment ofeach finger may also include at least one peripheral portion andcorresponding central cut-out portion. A fixation member component,according to some embodiments, includes the above described plurality offingers integrally formed with a base ring, wherein the fixed end ofeach fingers is joined to the base ring.

According to some system embodiments disclosed herein, a delivery toolcontains an entirety of the device while holding the first segment ofeach of the fixation fingers thereof in the extended condition, todeliver the device to a target implant site. Once in proximity to thesite, the tool may be manipulated to expose the fixation fingers andthereby release the first segment of each to the relaxed condition,after which the tool and device together may be advanced to the sitethereby wedging the exposed fingers between opposing tissue surfaces sothat the first segment of each is in the compressed condition. In thecompressed condition, the free tip of the above-described centralcut-out portion(s) of some or all of the fixation fingers may catch, orlodge against opposing tissue surfaces, via a spring force of thecompressed fingers, to provide fixation for the implanted device. Thesecond segment and each central cut-out portion of each fixation fingeris preferably configured to prevent penetration thereof within tissue atthe implant site.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals denote like elements,and:

FIG. 1 is a schematic diagram showing potential implant sites forembodiments of the present invention;

FIG. 2A is a perspective view of an implantable medical device,according to some embodiments;

FIGS. 2B-C are elevation and end views of an exemplary fixation membercomponent which may be employed by the device of FIG. 2, according tosome embodiments;

FIGS. 2D-E are elevation and end views of another exemplary fixationmember component which may be employed by the device of FIG. 2,according to some alternate embodiments;

FIG. 2F is an enlarged plan view of a finger of the fixation membercomponent of FIGS. 2D-E, according to some alternate embodiments;

FIG. 3 is a plan view of an interventional medical system with a partialcut-away section, according to some embodiments;

FIGS. 4A-D are schematics outlining some methods of the presentinvention;

FIG. 5A is a perspective view of an implantable medical device,according to some additional embodiments;

FIG. 5B is a cross-section view through a portion of the device of FIG.5A, according to an exemplary construction of some embodiments; and

FIGS. 6A-C are schematics according to some alternate methods of thepresent invention.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical examples, and those skilled in the art will recognize thatsome of the examples may have suitable alternatives.

FIG. 2A is a perspective view of an implantable medical device 200,according to some embodiments. FIG. 2A illustrates device 200 includinga hermetically sealed housing 205, preferably formed from abiocompatible and biostable metal such as titanium, which contains apulse generator (e.g., a power source and an electronic controller—notshown), a fixation member, which is formed by a plurality of fixationfingers 230 spaced apart from one another around a perimeter of a distalend 201 of housing 205, and an electrode 261, which is located at thedistal end 201 of housing 205 being coupled to the controller of device200 by a hermetic feedthrough assembly (not shown) constructed accordingto those known to those skilled in the art of implantable medicaldevices. Housing 205 may be overlaid with an insulative layer, forexample, medical grade polyurethane, parylene, or silicone, and FIG. 2Afurther illustrates another electrode 262 of device 200, which may beformed by removing a portion of the insulative layer to expose themetallic surface of housing 205. According to the illustratedembodiment, electrode 262 may function in conjunction with electrode 261for bipolar pacing and sensing, when fixation fingers 230 hold electrode261 in intimate tissue contact at a target implant site, for example,within right atrial appendage 102 or within right ventricle RV inproximity to apex 103 (FIG. 1). Fixation fingers 230 function to holddevice 200 at the implant site by being wedged between opposing tissuesurfaces at the site.

FIGS. 2B-C are elevation and end views of an exemplary fixation membercomponent 23 which may be employed by device 200, according to someembodiments. FIGS. 2B-C illustrate fixation member component 23including eight fixation fingers 203 integrally formed with one anotherand a base ring 239, such that a thickness t of base ring 239 isapproximately the same as that of each finger 230. According to anexemplary embodiment, fixation member component 23 is cut from Nitinoltubing, according to methods known in the art, and thickness t may be0.005 inch+/−0.001 inch, wherein base ring 239 may have an innerdiameter id of approximately 0.20 inch and an outer diameter od ofapproximately 0.21 inch. A height x of base ring 239 may beapproximately equal to a width w of each finger, for example,approximately 0.024 inch. After cutting the aforementioned Nitinoltubing, fingers 230 are shaped by bending and holding fingers 230 in theillustrated curvature while heat treating component 23 according tomethods known to those skilled in the art. FIG. 2B illustrates (viacross-section through section line B-B of FIG. 2C) each fixation finger230 including a first segment 231 and a second segment 232, wherein eachfirst segment 231 extends from a fixed end 235 of the correspondingfinger 230 to the corresponding second segment 232, and each secondsegment 232 extends from the corresponding first segment 231 to a freeend 236 of the corresponding finger 230. FIGS. 2A-B further illustrateseach first segment 231, in a relaxed condition, extending in an arc,distally and outwardly from fixed end 235, and second segment 232extending from first segment 231 in a proximal direction and outwardfrom device housing 205. With further reference to FIG. 2C fixationfingers 230 are spaced equally apart from one another such that an angleψ defined between each adjacent pair is approximately 45 degrees.Component 23 may be mounted to distal end 201 of device housing 205, forexample, in a manner similar to that described for a fixation component102 in co-pending and commonly assigned United States Patent Application2012/0172690, which description is hereby incorporated by reference.

According to the illustrated embodiment, first segment 231 of eachfixation finger 230 is elastically deformable between the relaxedcondition and an extended condition, per arrow E of FIG. 2B, and betweenthe relaxed condition and a compressed condition, per arrow C of FIG.2B. The extended condition is described below in conjunction with FIGS.3, 4A, and 6A-B; and the compressed condition is described below inconjunction with FIGS. 4C and 6C. With further reference to FIG. 2B, theangle enclosed by the arc of first segment 231 of each finger 230 isshown being at least 90 degrees, with second segment 232 extending awayfrom first segment 231 at an angle θ. According to an exemplaryembodiment, a radius r of the arc of each first segment 231 isapproximately 0.067 inch, and angle θ is approximately 26 degrees. FIG.2B further illustrates each second segment 232 extending in a proximaldirection from first segment 231 over a distance just slightly greaterthan a distance z, wherein distance z may be approximately 0.095 inchmeasured from a proximal edge of base ring 239 to a tangent lineextending from an intersection of first and second segments 231, 232.Although not shown in FIG. 2A, according to some preferred embodiment,electrode 261 may be mounted on a relatively short extension formed indistal end 201 of housing 205 such that electrode 261 is spaced distalto radius r of each finger 230, for example, as shown in FIGS. 3 and 4B,wherein the distance from the apex of radius r to the distally spacedelectrode 261 may be approximately 2 mm.

FIGS. 2D-E are elevation and end views of another exemplary fixationmember component 123 which may be employed by device 200 in lieu ofcomponent 23, according to some alternate embodiments. FIGS. 2D-Eillustrate fixation member component 123 including six fixation fingers1203 integrally formed with one another and a base ring 1239, such thata thickness t2 of base ring 1239 is approximately the same as that ofeach finger 1230. Fixation fingers 1230 are shown spaced equally apartfrom one another such that an angle π defined between each adjacent pairis approximately 60 degrees. A height x2 of base ring 1239 may beapproximately 0.02 inch and a width w2 of each finger 1230 may beapproximately 0.05 inch. FIGS. 2D-E further illustrate each finger 1230including a first segment 1231 and a second segment 1232, similar tofingers 230 of component 23, wherein each first segment 1231 extendsfrom a fixed end 1235 of the corresponding finger 1230 to thecorresponding second segment 1232, and each second segment 1232 extendsfrom the corresponding first segment 1231 to a free end 1236 of thecorresponding finger 1230. In contrast to fingers 230 of component 23,first segment 1231 of each finger 1230 includes a peripheral portion 281and a central cut-out portion 291, which is framed by peripheral portion281, as is best seen in the enlarged plan view of one of fingers 1230 inFIG. 2F.

With further reference to FIG. 2D, first segment 1231, in a relaxedcondition, is shown (via cross-section through section line D-D of FIG.2E) extending in a compound curve (e.g., having two radii of curvature,a first being approximately 0.065 inch and a second being approximately0.29 inch), distally and outwardly from the corresponding fixed end1235, and second segment 1232 is shown extending from first segment 1231in a proximal direction and outward from base ring 1239. According tothe illustrated embodiment, first segment 1231 of each fixation finger1230 is elastically deformable between the relaxed condition and anextended condition, per arrow E of FIG. 2D, and between the relaxedcondition and a compressed condition, per arrow C of FIG. 2D. Theextended condition is described below in conjunction with FIGS. 3, 4A,and 6A-B; and the compressed condition is described below in conjunctionwith FIG. 4D. With further reference to FIG. 2D, the compound curve offirst segment 1231 of each finger 230 encloses an angle φ that isbetween approximately 135 degrees and 180 degrees, and, according to anexemplary embodiment, a radius r2 of a first portion of the compoundcurve is approximately 0.065 inch, and a distance z2 from a proximaledge of base ring 1239 to an apex of the first portion of the compoundcurve is approximately 0.095 inch. Second segment 1232 of each finger230 may extend along a radius of curvature in an opposite direction, forexample, being approximately 0.29 inch, and starting at an inflectionpoint I, which is designated in FIG. 2D.

FIG. 2F illustrates peripheral portion 281 of first segment 1231, whichmay be likened to a strut, framing central cut-out portion 291 so thatan inside edge 81 of peripheral portion 281 is spaced apart from anoutside edge 91 of central cut-out portion 291. FIGS. 2E-F furtherillustrate each central cut-out portion 291 extending from a fixed end95 thereof to a free tip 96 thereof, wherein fixed end 95 is integralwith the corresponding peripheral portion 281 and located in proximityto fixed end 1235 of the corresponding finger 1230. With reference toFIG. 2F, a length L1 of central cut-out portion 291 extends along amajority of the length of first segment 1231 and may be approximately0.25 inch, according to some exemplary embodiments. FIG. 2F furtherillustrates central cut-out portion 291 tapering from a first width c1,at fixed end 95, to a second width c2, in proximity to free tip 96,wherein first width c1 is greater than second width c2; and free tip 96is shown being rounded and having a width f greater than second widthc2.

With further reference to FIGS. 2D-F, according to some embodiments,second segment 1232 of each finger 1230 includes two peripheral portions282A, 282B, each framing a corresponding central cut-out portion 292A,292B, which are spaced apart from one another along a length of secondsegment 1232. According to the illustrated embodiment, a width of eachcentral cut-out portion 292A, 292B is constant along the correspondinglength thereof, for example, being approximately 0.018 inch. A free tip96A of each central cut-out portion 292A is preferably located along theaforementioned curvature of the corresponding second segment 1232, asmay be seen in FIG. 2D, for example, such that a distance A (FIG. 2F)between each free tip 96A and fixed end 1235 of the corresponding finger1230 may be approximately 0.4 inch, wherein a length L2 of each portion292A may be approximately 0.1 inch. The other, distal-most, centralcut-out portion 292B of each finger 1230 is relatively shorter andlocated in close proximity to the corresponding free end 1236, forexample, to reduce a stiffness of each finger 1230 at the free end 1236thereof, and thereby preventing tissue penetration. With furtherreference to FIG. 2F, a distance D between an inner edge 82A ofperipheral portion 282A and central cut-out portion 292B can dictate astiffness of portion 292B (the smaller D, the less stiff portion 292B),and, according to some exemplary embodiments, distance D isapproximately 0.015 inch.

Fixation member component 123, like component 23, may be cut fromNitinol tubing, according to methods known in the art, and thickness t2may be 0.005 inch+/−0.001 inch, wherein base ring 1239 may have the sameinner diameter id (approximately 0.20 inch) and outer diameter od(approximately 0.21 inch) as base ring 239 of component 23, for example,being sized for mounting around a perimeter of device housing 205, inlieu of component 23 and in a similar manner to that described in theabove-referenced application '690. After cutting the Nitinol tubing,fingers 1230 are shaped by bending and holding fingers 1230 in theillustrated curvature while heat treating component 123 according tomethods known to those skilled in the art.

FIG. 3 is a plan view of an interventional medical system with a partialcut-away section, according to some embodiments, wherein the systemincludes a delivery tool 300, in which device 200 is loaded, fordeploying device 200 to a target implant site. FIG. 3 illustrates tool300 including a handle 310, an elongate inner member 320, and an outerassembly, which is formed by an elongate deployment tube 330 and anouter, stabilizing sheath 370 that is secured to handle 310 andsurrounds a proximal portion of deployment tube 330 in proximity tohandle 310. According to the illustrated embodiment, elongate innermember 220 extends within a lumen 335 of deployment tube 330, and aproximal end of deployment tube 330 is coupled to a control member 312of handle 310 such that an entirety of deployment tube 330 is movablewith respect to the inner member 320, via control member 312. FIG. 3further illustrates inner member 320 including a distal end 322, whichis located within a distal-most portion 332 of deployment tube 330, andwhich is configured to engage implantable medical device 200 by abuttingproximal end 202 of device housing 205, as shown.

With further reference to FIG. 3, that portion of deployment tube lumen335 which extends along a length of distal-most portion 332 is sized tocontain distal end 322 of inner member 320 together with an entirety ofdevice 200. FIG. 3 shows fixation fingers 230/1230 of the loaded device200 being held by distal-most portion 332 in the aforementioned extendedposition. With reference to FIG. 4A, a distal portion of tool 300, withan entirety of device 200 loaded in distal-most portion 332, may benavigated to a target implant site, for example, in the right atrium RA(or right ventricle RV), by advancing tool 300 through a venous systemof the patient, for example, from a femoral venous access site and upthrough the inferior vena cava IVC. A length of deployment tube 330,between handle 310 and a distal opening 303 of deployment tube 330, whentube 330 is in the position shown in FIG. 3, may be betweenapproximately 103 cm and approximately 107 cm, for example, to reach theright atrium RA from the femoral access site. According to someembodiments of the present invention, delivery tool 300 includesarticulating features to facilitate the navigation of the distal portionof delivery tool 300; for example, inner member 320 of delivery tool 300may include a pull wire (not shown) integrated therein and coupled toanother control member 311 of handle 310 that, when moved per arrow A,causes inner member 320 and deployment tube 330 to bend along distalportions thereof. Suitable construction detail for a delivery tool liketool 300 is described in co-pending and commonly assigned U.S. patentapplication Ser. No. 14/039,937, the description of which is herebyincorporated by reference.

According to some methods of the present invention, once an operator haslocated distal-most portion 332 in a chamber of the heart, for example,the right atrium RA, as shown in FIG. 4A, the operator can retractdeployment tube 330, per arrow W (FIG. 3), for example, by movingcontrol member 312 per arrow B (FIG. 3), to release fixation fingers230/1230 to the relaxed position. Fingers 230 are shown in FIG. 4B, butif fingers 1230 were substituted for fingers 230, the relaxed positionthereof would be like that shown in FIG. 2D. FIG. 4B illustrates eachfinger 230 having been exposed out through distal opening 303 ofdeployment tube 330 so that, in the relaxed position, each finger 230extends in a proximal direction and outward from device housing 205.After releasing device fixation fingers 230/1230, the operator mayadvance tool 300 and device 200 together to a target implant sitebetween folds of tissue, for example, pectinate muscle bands in rightatrial appendage 102, and, thus, wedge the exposed fixation fingers230/1230 between opposing tissue surfaces as shown schematically inFIGS. 4C-D. It should be noted that, with further reference to FIG. 4A,an alternate implant site may be in the right ventricle RV, wherefixation fingers 230 may be wedged between folds of tissue (trabeculae)in the area of apex 103.

With reference to FIGS. 4C-D, distal end 322 of device inner member 320may be employed to provide a push force that assists in wedging fingers230/1230 so that fingers 230/1230 are in the aforementioned compressedstate to hold electrode 261 in intimate tissue contact. The compressedfingers 230/1230, having a super-elastic nature, hold device 200 inplace at the implant site by a spring force (per the bold arrows of FIG.4C), and that finger free ends 236/1236 are preferably configured toprevent penetration thereof within tissue at the implant site, whilemerely catching, or lodging against opposing tissue surfaces. Theinclusion of distal-most central cut-out portion 292B in each finger1230, as was described above in conjunction with FIG. 2F, is an exampleof a configuration that can prevent finger penetration. With referenceback to FIG. 2, according to some embodiments, one or more of fingerfree ends 236 includes a discrete radiopaque marker 238 attachedthereto, for example, a platinum-iridium rivet like member, that mayassist the operator in assessing the fixation of device 200 at theimplant site. Furthermore, it should be noted that the fixation fingers230 or 1230, as described above in conjunction with FIGS. 2B-F, may alsobe formed from a polymer material, either individually or integrallywith the corresponding base ring 239, 1239, wherein an appropriatepolymer material and associated dimensional specifications essentiallymimics that of fingers 230, 1230 formed from the aforementioned Nitinol,in terms of spring properties.

With reference to FIG. 4D, fixation fingers 1230, substituted forfingers 230 in device 200, may be more effective to hold electrode 261and device 200 in place when a depth of the fold, or crease betweenopposing tissue surfaces at the implant site is relatively shallow. Avariety of depths may be encountered depending upon anatomical variationfrom patient to patient or from one potential implant site to anotherwithin a particular patient. FIG. 4D illustrates free tips 96 of centralcut-out portions 291 of opposing fingers 1230, in the compressedcondition, catching, or lodging against opposing tissue surfaces in afold of tissue that may not be deep enough to accommodate a full lengthof fingers 230. But, even in a deeper crease or fold, the configurationof each finger 1230, according to some embodiments, provides multiplefree ends or tips per finger to catch, or lodge against opposing tissuesurfaces for enhanced fixation when finger 1230 is in the compressedcondition. As was described above, in conjunction with FIG. 2F, in theseembodiments, second segment 1232 of each finger 1230 includes centralcut-out portion 292A and corresponding peripheral portion 282A, whichmay provide potential for a greater force of fixation for device 200 atan implant site, if the depth thereof between folds of tissue issufficient for free tip 96A of portion 292A to catch, or lodge againstthe opposing tissue surfaces when fingers 1230 are wedged therebetween.

With further reference to FIG. 2F, the above-described taperingconfiguration of each central cut-out portion 291, in conjunction withthe enlarged free tip 96 thereof, helps to prevent the penetrationthereof into tissue at the implant site; furthermore, the configurationmay enhance a fatigue life of fingers 1230 during chronic implantationof device 200 with fingers 1230 in the compressed condition.Furthermore, it should be noted that the fixation provided by fingers1230 may be more aggressive if a spacing between outer edges 91, 92A andcorresponding inner edges 81, 82A is relatively large. But moreaggressive fixation may lead to more tissue in-growth over time and,thus, more difficulty in extracting device 200 after chronicimplantation, therefore, a relatively smaller spacing between theaforementioned edges may be desired to provide a balance of adequatefixation and ease of chronic extraction. It is contemplated that one ormore of inner edges 81, 82A, 82B of each finger 1230 could be tailoredto cut through tissue in-growth as device 200 is extracted. It shouldalso be noted that if the exemplary dimensions presented above inconjunction with FIGS. 2B-F are scaled down, for example, in proportionto a smaller overall implantable device volume, they will still fallwithin the scope of embodiments of the present invention.

Returning now to FIGS. 4C-D, after wedging fingers 230/1230 betweenopposing tissue surfaces, the operator may evaluate pacing performanceof electrode 261 before completely withdrawing delivery tool 300 awayfrom the implanted device 200. Thus, if the operator determines that theperformance is not satisfactory, the operator may advance distal-mostportion 332 of deployment tube 330 back in a distal direction, forexample, via control member 312 (FIG. 3), relative to device 200 andinner member 320, and over wedged fixation fingers 230/1230 to movedevice 200 back into distal-most portion 332 with fingers 230/1230 movedback into the extended condition, as shown in FIG. 3. Then the operatorcan move delivery tool 300 with the re-loaded device 200 into proximitywith an alternative implant site, retract deployment tube 330 again toexpose and release fingers 230/1230 into the relaxed condition (FIG.4B), and then advance tool 300 toward the other site to wedge theexposed fingers 230/1230 between opposing tissue surfaces at the othersite.

FIG. 5A is a perspective view of an implantable medical device 500,according to some additional embodiments; and FIG. 5B is a cross-sectionview through a portion of device 500, according to an exemplaryconstruction of some embodiments. FIG. 5A illustrates device 500 beingsimilar to device 200 but including a pacing extension 560 on which apacing electrode 561 is mounted, in lieu of electrode 261 of device 200.FIG. 5A further illustrates extension 560 including a preformedcurvature located in proximity to, and proximal to electrode 561. Adiameter of extension 560 may be approximately 0.05 inch (1.3 mm); anoverall length of extension 560 may be approximately 0.6 inch (15 mm);and the curvature, preferably in a single plane, is defined by a radiusR, which may be approximately 0.2 inch, according to an exemplaryembodiment. According to the illustrated embodiment, electrode 561 islocated in close proximity to a distal tip 565 of extension 560, whichtip 565 is preferably tapered. FIG. 5B illustrates distal tip 565 beingslightly enlarged from a remainder of extension 560; and, according tosome embodiments, tip 565 includes electrode 561, which forms at least aportion of the taper, and a relatively soft medical grade siliconerubber member 567, which may include a steroid embedded therein. Theillustrated contour of electrode 561 may help electrode 561 to makebetter tissue contact when tip 565 lies adjacent to tissue, for example,as illustrated in FIG. 6C. According to an exemplary embodiment, adiameter of tip 565 (as shown in FIG. 5B) is approximately 0.07 inch(1.8 mm), and a surface area of electrode 561 is approximately 5.8 mm².Electrode 561 may be formed from a platinum iridium alloy.

FIG. 5B further illustrates pacing extension 560 being formed by acoiled multi-filar conductor 562 (e.g., MP35N alloy) enclosed within ajacket of insulation 564 (e.g., medical grade polyurethane), and anexemplary junction between electrode 561 and conductor 562, which may besecured by crimping and/or welding according to methods known in the artof implantable medical electrical leads. According to the illustratedembodiment, conductor 562 electrically connects electrode 561 to theaforementioned pulse generator contained within device housing 205, forexample, via a feedthrough assembly constructed according to methodsknown in the art of implantable medical devices.

Turning now to FIG. 6A, as was described above for device 200, device500 is loaded into distal-most portion 332 of delivery tool 300 (FIG.3), such that fixation fingers 230 are in the extended condition. Itshould be noted that fixation fingers 1230 may be substituted forfingers 230 in alternate embodiments. FIG. 6A illustrates fingers230/1230 extending in a distal direction and alongside pacing extension560 within distal-most portion 332. After device 500 is loaded, theoperator may navigate delivery tool 300, with device 500 completelycontained therein, through the patient's venous system, for example,from a femoral venous access site, up through the inferior vena cavaIVC, and into a chamber of the heart, for example, the right atrium RA,as shown in FIG. 4A. In some preferred embodiments, pacing extension 560extends distally beyond extended fingers 230/1230 so that the operatormay withdraw deployment tube 330, per arrow W, just enough to exposeelectrode 561 out through distal opening 303 thereof, as shown in FIG.6B. According to some exemplary embodiments, extension 560 can extendapproximately 2 to 4 mm beyond opening 303 without fingers 230/1230being exposed, so that the operator can advance tool 300 to one or morepotential implant sites, where electrode 561 makes contact, to mapelectrical activity and/or to check pacing thresholds. According to somemethods, after finding a desired implant site in this manner, theoperator can pull back tool 300 and device 500 together, for example, tothe position shown in FIG. 6B, and then retract deployment tube 330 evenfurther, with respect to device 500 and inner member 320, to exposefixation fingers 230/1230 out from distal opening 303, thereby releasingfingers 230/1230 to the relaxed condition, for example, as illustratedin FIG. 4B. Then, as described above, the operator can advance tool 300and device 500 together back to the desired implant site, for example,between pectinate muscle bands in right atrial appendage 102, and, thus,wedge the exposed fixation fingers 230/1230 between opposing tissuesurfaces, as shown schematically in FIG. 6C for fingers 230, to holddevice 500 at the implant site with electrode 561 making intimate tissuecontact. With further reference to FIG. 6C, it may be appreciated thatthe length of pacing extension 560 serves to separate that portion ofthe implant site at which electrode 561 makes contact with that portionof the site at which fixation fingers 230 make spring contact (e.g., perbold arrows), so that any inflammation associated with the fixationfingers contact may not impair chronic pacing thresholds.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims.

We claim:
 1. An interventional medical system comprising: an implantablemedical device comprising an electronic controller, a hermeticallysealed housing containing the controller, a pacing electrodeelectrically coupled to the controller and mounted in proximity to adistal end of the housing, and a fixation member mounted to the distalend of the housing, the fixation member comprising a plurality offingers spaced apart from one another around a perimeter of the distalend of the housing, and each finger comprising: a first segmentextending from a fixed end of the corresponding finger and beingelastically deformable between a relaxed condition and an extendedcondition, and between the relaxed condition and a compressed condition,and each first segment including a peripheral portion and a centralcut-out portion, the peripheral portion framing the central cut-outportion and having an inside edge spaced apart from an outside edge ofthe central cut-out portion, and the central cut-out portion extendingfrom a fixed end thereof to a free tip thereof, the fixed end of thecentral cut-out portion being integral with the peripheral portion inproximity to the fixed end of the corresponding finger, and the centralcut-out portion being configured to prevent penetration thereof withintissue at the implant site; and a second segment extending from thecorresponding first segment to a free end of the corresponding finger,the second segment being configured to prevent penetration thereofwithin tissue at an implant site, the second segment extending from thecorresponding first segment in a proximal direction and outward from thedevice housing, when the first segment is in the relaxed condition, andthe second segment extending distally from the distal end of the devicehousing, when the first segment is in the extended condition; and adelivery tool comprising a handle, an elongate inner member and adeployment tube, the handle including a control member, the elongateinner member including a proximal end secured to the handle and a distalend configured to abut a proximal end of the device housing, and thedeployment tube including a proximal end coupled to the control memberof the handle, a distal-most portion, and a lumen extending from aproximal opening thereof to a distal opening thereof, the proximalopening being located in proximity to the proximal end of the deploymenttube, and the distal opening terminating the distal-most portion, andthe inner member extending within the lumen of the deployment tube suchthat the deployment tube is movable with respect to the inner member viathe control member of the handle; and wherein the distal-most portion ofthe deployment tube of the tool is sized to contain the distal end ofthe inner member together with an entirety of the device when the distalend of the inner member abuts the proximal end of the device housing andwhen the first segment of each finger of the fixation member is in theextended condition, being held in the extended condition by thedistal-most portion.
 2. The system of claim 1, wherein the centralcut-out portion of the first segment of each finger of the devicefixation member tapers from a first width at the fixed end thereof to asecond width in proximity to the free tip thereof, the first width beinggreater than the second width, and the free tip being rounded and havinga width greater than the second width.
 3. The system of claim 1, whereinthe first segment of each finger of the device fixation member, when thefinger is in the relaxed condition, extends in a compound curve,distally and outwardly from the corresponding fixed end, the curveenclosing an angle between approximately 135 degrees and 180 degrees. 4.The system of claim 1, wherein the plurality of fingers of the devicefixation member comprises six fingers spaced equally apart from oneanother around the perimeter of the device housing.
 5. The system ofclaim 1, wherein: the second segment of each finger of the devicefixation member includes at least one peripheral portion and acorresponding central cut-out portion, each peripheral portion framingthe corresponding central cut-out portion and having an inside edgespaced apart from an outside edge of the corresponding central cut-outportion; and each central cut-out portion of each second segment extendsfrom a fixed end thereof to a free tip thereof in a direction toward thefree end of the corresponding finger, the fixed end of each centralcut-out portion being integral with the corresponding peripheralportion, and each central cut-out portion being configured to preventpenetration thereof within tissue at the implant site.
 6. The system ofclaim 5, wherein the central cut-out portion of the first segment ofeach finger of the device fixation member tapers from a first width atthe fixed end thereof to a second width in proximity to the free tipthereof, the first width being greater than the second width, and thefree tip being rounded and having a width greater than the second width.7. The system of claim 5, wherein the first segment of each finger ofthe device fixation member, when the finger is in the relaxed condition,extends in a compound curve, distally and outwardly from thecorresponding fixed end, the curve enclosing an angle betweenapproximately 135 degrees and 180 degrees.
 8. The system of claim 5,wherein the at least one peripheral portion and corresponding centralcut-out portion of the second segment of each finger of the devicefixation member comprises two peripheral portions and correspondingcentral cut-out portions spaced apart from one another along a length ofthe second segment.
 9. The system of claim 5, wherein the plurality offingers of the device fixation member comprises six fingers spacedequally apart from one another around the perimeter of the devicehousing.
 10. The system of claim 1, wherein the device further includesa pacing extension on which the pacing electrode is mounted, the pacingextension extending distally from the distal end of the housing andelectrically coupling the pacing electrode to the controller.
 11. Thesystem of claim 10, wherein: the pacing electrode of the device islocated in close proximity to a distal tip of the pacing extension ofthe device; and the pacing extension of the device includes a pre-formedcurvature in proximity to and proximal to the pacing electrode.
 12. Thesystem of claim 10, wherein the pacing electrode of the device islocated distal to the free end of each finger of the device fixationmember, when the proximal segment of each fixation finger is in theextended condition.
 13. A relatively compact implantable medical devicecomprising an electronic controller, a hermetically sealed housingcontaining the controller, a pacing electrode electrically coupled tothe controller and mounted in proximity to a distal end of the housing,and a fixation member mounted to the distal end of the housing, thefixation member comprising a plurality of fingers spaced apart from oneanother around a perimeter of the distal end of the housing, and eachfinger comprising: a first segment extending from a fixed end of thecorresponding finger and being elastically deformable between a relaxedcondition and an extended condition, and between the relaxed conditionand a compressed condition, and each first segment including aperipheral portion and a central cut-out portion, the peripheral portionframing the central cut-out portion and having an inside edge spacedapart from an outside edge of the central cut-out portion, and thecentral cut-out portion extending from a fixed end thereof to a free tipthereof, the fixed end of the central cut-out portion being integralwith the peripheral portion in proximity to the fixed end of thecorresponding finger, and the central cut-out portion being configuredto prevent penetration thereof within tissue at the implant site; and asecond segment extending from the corresponding first segment to a freeend of the corresponding finger, the second segment being configured toprevent penetration thereof within tissue at an implant site, the secondsegment extending from the corresponding first segment in a proximaldirection and outward from the device housing, when the first segment isin the relaxed condition, and the second segment extending distally fromthe distal end of the device housing, when the first segment is in theextended condition.
 14. The device of claim 13, wherein the centralcut-out portion of the first segment of each finger of the fixationmember tapers from a first width at the fixed end thereof to a secondwidth in proximity to the free tip thereof, the first width beinggreater than the second width, and the free tip being rounded and havinga width greater than the second width.
 15. The device of claim 13,wherein the first segment of each finger of the fixation member, whenthe finger is in the relaxed condition, extends in a compound curve,distally and outwardly from the corresponding fixed end, the curveenclosing an angle between approximately 135 degrees and 180 degrees.16. The device of claim 13, wherein the plurality of fingers of thefixation member comprises six fingers spaced equally apart from oneanother around the perimeter of the housing.
 17. The device of claim 13,wherein: the second segment of each finger of the device fixation memberincludes at least one peripheral portion and a corresponding centralcut-out portion, each peripheral portion framing the correspondingcentral cut-out portion and having an inside edge spaced apart from anoutside edge of the corresponding central cut-out portion; and eachcentral cut-out portion of each second segment extends from a fixed endthereof to a free tip thereof in a direction toward the free end of thecorresponding finger, the fixed end of each central cut-out portionbeing integral with the corresponding peripheral portion, and eachcentral cut-out portion being configured to prevent penetration thereofwithin tissue at the implant site.
 18. The device of claim 17, whereinthe central cut-out portion of the first segment of each finger of thefixation member tapers from a first width at the fixed end thereof to asecond width in proximity to the free tip thereof, the first width beinggreater than the second width, and the free tip being rounded and havinga width greater than the second width.
 19. The device of claim 17,wherein the first segment of each finger of the fixation member, whenthe finger is in the relaxed condition, extends in a compound curve,distally and outwardly from the corresponding fixed end, the curveenclosing an angle between approximately 135 degrees and 180 degrees.20. The device of claim 17, wherein the at least one peripheral portionand corresponding central cut-out portion of the second segment of eachfinger of the fixation member comprises two peripheral portions andcorresponding central cut-out portions spaced apart from one anotheralong a length of the second segment.
 21. The device of claim 17,wherein the plurality of fingers of the fixation member comprises sixfingers spaced equally apart from one another around the perimeter ofthe housing.
 22. The device of claim 13, further comprising a pacingextension on which the pacing electrode is mounted, the pacing extensionextending distally from the distal end of the housing and electricallycoupling the pacing electrode to the controller.
 23. The device of claim22, wherein: the pacing electrode is located in close proximity to adistal tip of the pacing extension; and the pacing extension includes apre-formed curvature in proximity to and proximal to the pacingelectrode.
 24. The device of claim 22, wherein the pacing electrode islocated distal to the free end of each finger of the fixation member,when the proximal segment of each fixation finger is in the extendedcondition.